Although the laser was discovered only 25 years ago, its applications in science, medicine and industry have made a significant impact on our society. As a result, the sales of commercial laser systems have increased rapidly and are now at a level of nearly a half billion dollars annually. This explosive growth is typified by the rare gas-halide (RGH) excimer lasers. Although discovered just a few years ago, the sales of excimer laser systems already represent one of the two fastest growing segments of the electro-optics market.
While most of the units sold presently are intended for laboratory use, the engineering sophistication now incorporated into these lasers has made them quite rugged and reliable and they are being increasingly used for industrial processes such as silicon wafer marking and laser annealing of implanted semiconductor layers. Xenon chloride (XeCl) is, by far, the workhorse of the RGH laser family. Its wavelength of 308 nm is ideally suited for "pumping" dye lasers for laboratory use or for the semiconductor applications mentioned above.
One of the main reasons for the popularity of these lasers is the efficiency with which they generate ultraviolet (UV) radiation. Typical discharge pumped, rare gas-halide lasers exhibit a conversion efficiency (stored electrical energy to laser output energy) in the range of 1-3%. While appearing to be small, such efficiencies are, in fact, quite large for UV lasers. Nevertheless, it is the laser's efficiency which primarily determines its size, weight and cost. Therefore, even a modest improvement in the efficiency of such lasers would have tremendous economic implications.
It has been found that the efficiency of a pulsed laser system can be increased by more than 50% by injecting radiation into the system, generally 20 to 200 nanoseconds prior to the emergence of the system's laser pulse. The potential impact of this development is great. For specific laser systems, it now becomes possible to either increase the output power of a laser device of a given size or to decrease system dimensions, weight and cost while maintaining a constant power output.
While broadly applicable to pulsed laser systems, (gaseous, liquid and solid state), the system of this invention finds a preferred utility with the rare gas halide excimer lasers [see, for example, Excimer Lasers, 2nd edition, edited by C. K. Rhodes (Springer, 1984)]. It should be noted that this invention is fundamentally different from injection locking, a well-understood phenomenon. For example, the purpose of injection locking is to improve the optical quality of the output beam of a large laser (known as the "amplifier") with radiation from a low power laser "oscillator". This technique involves operating the oscillator and amplifier at essentially the same wavelength. Also, high optical quality laser beams are those that diverge slowly (i.e., at one to three times the diffraction limit) and/or have narrow line widths. If the "seed" laser pulse from the oscillator is injected into the amplifier 10-20 nanoseconds before the amplifier produces its laser pulse, then the high optical quality of the oscillator pulse will be replicated by the amplifier. However, two points need to be emphasized with regard to injection locking:
(1) there is little difference in the output power of the amplifier with or without the "seed" pulse from the oscillator; and
(2) the ability to "lock" the amplifier is critically dependent on the time delay between the firing of the oscillator and amplifier (or, equivalently, the generation of the two optical pulses). Time delays greater than roughly 25 ns are ineffective and 15-20 ns is optimum. Consequently, the invention described here cannot be attributed to injection-locking. Typical of injection locking devices is the Model EMG 150 ET excimer laser system manufactured by Lambda Physik GmbH of Goettingen, Federal Republic of Germany.
Many pulsed gas lasers, particularly those that either operate at high pressures or employ strongly-attaching gases, require preionization of the active medium to achieve stable operation. That is, before the main discharge occurs, it is necessary to produce a spatially uniform concentration of 10.sup.7 -10.sup.9 electrons per cm.sup.3. Otherwise, the discharge will rapidly collapse into an arc. Lasers requiring such preionization generally contain a built-in preionizer that generates (by a spark or corona) incoherent ultraviolet radiation. As it propagates through the gaseous active medium, this radiation photoionizes various atomic and molecular species, thus producing the necessary electrons. A substantial fraction of the radiation produced by a spark preionizer, for example, has wavelengths between 150 and 200 nm. Such photons are sufficiently energetic to ionize most of those atoms and molecules ordinarily necessary for the operation of the laser. The excimer lasers, for example, involve one or more of the rare gases and a halogen-containing molecule. Normally, pulsed lasers also contain trace concentrations of hydrocarbons due to the backstreaming of oil from the vacuum pump, the poor base pressure in the system (.about.10.sup.-3 Torr) or from chemical reactions with the laser chamber. In any case, the radiation from the internal preionizer is sufficiently energetic to produce an adequate photoelectron density by ionizing the rare gases, hydrocarbons, etc. without the need to introduce a low-ionization potential impurity into the gas mixture.
Preionization of one laser with radiation from a second laser has also been demonstrated previously. As reported by Taylor, et al., in Optics Letters, vol. 5, p. 216 (1980), the external laser radiation (having a wavelength different from that of the "main" laser) improves the uniformity of the active medium.
However, because excimer laser radiation at 193 nm or 248 nm (photon energies of 6.4 and 5.0 eV, respectively) is used to preionize the active medium of a second laser and because the second laser contained no preionizer, the external radiation is not effective as a preionization source unless a readily-ionizable impurity is intentionally added to the gas mixture. Taylor, et al., added flurobenzene to the XeCl gas mixture which can be photoionized at 248 nm. Without the fluorocarbon additive, the authors note that the XeCl laser output is approximately the same with external laser preionization as it is with an internal spark array preionizer.
In summary, using the longer wavelength radiation from an excimer laser is effective for preionization only when an impurity (having an ionization potential no greater than the excimer photon energy) is intentionally added to the gas mixture for the second laser. The system of this invention does not involve the addition of an impurity to the gas mixture, significantly enhances the output power of a pulsed laser for time delays much shorter than the smallest one reported by Taylor, et al., (70 ns) and, perhaps most importantly, works best when the wavelength of the external radiation is considerably greater than those studied by Taylor, et al. Therefore, preionization of the second laser by the external radiation plays a minor role in the system of this invention, particularly for external radiation wavelengths exceeding approximately 250 nm.